mid-14c., "support, base," from Old French sourse "a rising, beginning, fountainhead of a river or stream" (12c.), fem. noun taken from past participle of sourdre "to rise, spring up," from Latin surgere "to rise, arise, get up, mount up, ascend; attack," contraction of surrigere, from assimilated form of sub "up from below" (see sub-) + regere "to keep straight, guide" (from PIE root *reg- "move in a straight line," with derivatives meaning "to direct in a straight line," thus "to lead, rule"). Meaning "a first cause" is from late 14c., as is that of "fountain-head of a river." Meaning "person or written work supplying information or evidence" is by 1777.